Super-porous nanocomposite PNIPAm hydrogels reinforced with titania nanoparticles, displaying a very fast temperature response as well as pH-sensitivity

摘要

Super-porous nanocomposite hydrogels, based on PNIPAm matrix and reinforced by acidand base-resistant TiO_2 nanoparticles were prepared via freezing polymerization. The hydrogels display a fast swelling response to temperature (potential application as mechanical actuators), which is enabled by the nano-reinforcement. The reinforcing effect of TiO_2 was strongly enhanced (by nearly one order) through the incorporation of small amounts (2.5 mol%) of sodium methacrylate (SMA) as comonomer, which strongly adsorbs on the TiO_2 particles' surface, in analogy to ionic double layer adsorption of salts on free TiO_2 colloid particles. The SMA adsorption is so strong, that the pH-responsivity of the swelling of the PNIPAm-SMA copolymer is practically suppressed. A significant pH-responsivity was found only in such poly(NIPAm-co-SMA)/TiO_2 gels, which contained high amounts of chemical crosslinker. In these rather rigid gels, some of the SMA units were topologically prevented from adsorption to TiO_2 surface.